With the wide application of a liquid chromatography-mass spectrometry system analyzing complex mixture, ion sources operating in an atmospheric pressure, such as an electrospray ion source and an atmospheric pressure chemical ionization source, are critical to fields such as food safety, environmental protection and national security. However, in the analysis process, preprocessing before the samples are introduced into the analysis system costs a great deal of time, hindering real-time and fast operations of the analysis technologies in the industrial fields. This problem is partially solved with the appearance of some advanced direct analysis methods such as Desorption Electrospray Ionization (DESI, Science, vol. 306, P471 (2004)) and Direct Analysis in Real Time (DART, Analytical Chemistry, vol. 77, P2297 (2005)).
Afterwards, some other sample direct analysis technologies under the atmospheric pressure also achieved certain success, for example, ionization technology using heated gas for sample desorption includes an Atmospheric Solid Analysis Probe (ASAP), a Desorption Corona Beam Ionization and a Desorption Atmospheric Pressure Photoionization (DAPPI) technology introduced respectively in Analytical Chemistry, vol. 77, P7826-P7831 (2005), and Analytical Chemistry, vol. 79, P7867-P7872 (2007), in which, the former two uses corona discharge, while the latter one uses vacuum-ultraviolet light to assist ionization, and particularly analyzes non-polar and low-polar small molecule which is hard to be ionized through DESI successfully.
Meanwhile, in order to increase spatial resolution of a section where the sample is desorbed, an atmospheric pressure direct analysis method (Electrospray-assisted laser desorption ionization (ELDI)) using an ultraviolet laser as a desorption tool and using an electrospray plume as an ionization tool is introduced in Rapid Communication in Mass Spectrometry, vol. 19, P3701-P3704 (2005). In the method, since the laser is used as a desorption source, the desorption area of the surface of the sample can be well controlled, which enables imaging studies with mass spectrometry under atmospheric pressure. The similar technologies based on IR laser desorption technology such as laser desorption chemical ionization (LDCI) technology (using an atmospheric pressure chemical ionization source as the ion source) and laser desorption photoionization (LDPI) technology (using an atmospheric pressure photoionization source as the ion source) have been described in Rapid Communication in Mass Spectrometry, vol. 16, P681-P685 (2002) and in China patent (application No. 200810033974.4), respectively. And these sources are complementary to the above ELDI method due to their ionization capabilities on low-polar molecules.
However, since a separation step is removed before ionizing the complex mixture, the probability of mass spectrum peaks overlapping for different constituents is greatly increased, which produces difficulties in spectrum analysis. Taking the ionization method using heating gas for thermal desorption for example, when high heating temperature is used, each constituent of the mixture will be desorbed at the same time, causing congestion of mass spectrum peaks for different constituents. Meanwhile, as for the constituent capable of being desorbed in low temperature, the sample may be rapidly exhausted in high desorbing temperature, thereby affecting analysis efficiency of a mass spectrometer for the constituent. Therefore, how to separate a mixture during ionization to a certain extent or desorb the mixture with special focus on certain chemicals in order to improve the detection efficiency and accuracy of the mass spectrometer for a sample mixture is an issue in need of urgent solution.